In many applications, such as in the aircraft or aerospace industry, there is a need to generate electrical power for components. Because synchronous generators are typically used in these applications and are driven by variable speed prime movers (such as an aircraft jet engine) variable or wild frequency power is readily available. Typically, electrical power frequency spans a 2 to 1 range owing to the speed range of the prime mover. Because variable frequency power causes difficulties for certain types of loads, it is common to rectify the variable frequency power to obtain DC power. This power is thereafter filtered and used directly for DC loads or is inverted to AC power at a fixed frequency for constant frequency AC loads. Rectification, however, results in the generation of harmonic currents. In many applications, the magnitudes of these harmonic currents must be limited in order to prevent disturbance to other loads on the system. A filter of some type is usually necessary to reduce the harmonic content to a sufficient degree.
The design of the filter is complicated owing to the variability of harmonic content with generator frequency. Because of the need to design the filter to handle a wide range of harmonic frequencies, a size, weight and performance penalty is incurred.
One solution to the foregoing problem is to double the number of supply phases for rectification. This has been accomplished in the past by utilizing a twelve-pulse converter which converts a three-phase output of the generator to six-phase power. The converter includes an isolation transformer having wye and delta connected secondary windings which are coupled to a DC bus by rectifier circuits and an interphase reactor. Such an approach raises the frequencies of the harmonics in the rectified DC power and reduces the amplitudes of the lowest order harmonics making them easier to filter. However, a system including such a transformer is undesirable owing to the size and weight thereof.
A paper entitled "Polyphase Transformer Arrangements With Reduced KVA Capacities For Harmonic Current Reduction In Rectifier Type Utility Interface" by Choi et al., IEEE Paper dated June, 1995, pages 353-359, discloses a twelve-pulse power supply system utilizing an autotransformer in place of the transformer having wye and delta connected secondaries. The autotransformer includes secondaries that are coupled through rectifier circuits to first and second interphase transformers which produce the DC power. The autotransformer parts can have reduced ratings, resulting in a component which is physically smaller, less costly and more efficient than the conventional transformer described above.